1. Field of the Invention
The present invention relates to a zoom lens system and, more particularly though not exclusively, to an image pickup apparatus having the zoom lens system.
2. Description of the Related Art
In recent years, image pickup apparatuses, for example, a video camera, a digital still camera, and a broadcast camera, which use solid-state image sensing devices, and a camera using a silver-halide film have realized high level functions. Also, the image pickup apparatuses entire size has been miniaturized. A zoom lens, which has the features of being compact, short in overall length, and has a high resolution, would be useful as a shooting (photographic) optical system used in an image pickup apparatus.
What is called a rear-focus type zoom lens adapted to perform focusing by moving lens units, which are other than a first lens unit provided at an object side, is known as a zoom lens that has the features mentioned above (see Japanese Patent Application Laid-Open No. 7-270684 (corresponding to U.S. Pat. No. 5,963,378) and Japanese Patent Application Laid-Open No. 11-305124 (corresponding to U.S. Pat. No. 6,166,864)).
Generally, to enhance accommodatability of a camera in a nonused state (or no-image-taking state), it is effective to collapse (retract) each lens unit.
However, a 4-unit zoom lens of the zoom type discussed in Japanese Patent Application Laid-Open Nos. 7-270684 and 11-305124, where a second lens unit from an object side performs most of a function of varying a focal length, is unsuitable for a collapsible (retractable) structure, because image displacement sensitivity versus eccentricity of the first and second lens units is too high.
On the other hand, a 4-unit zoom lenses suitable for a collapsible structure are discussed in Japanese Patent Application Laid-Open No. 10-62687 (corresponding to U.S. Pat. No. 6,016,228), Japanese Patent Application Laid-Open No. 2001-194586 (corresponding to U.S. Pat. No. 6,456,441), Japanese Patent Application Laid-Open No. 2003-315676 (corresponding to U.S. Pat. No. 6,975,461), and Japanese Patent Application Laid-Open No. 2004-94233 (corresponding to U.S. Pat. No. 6,853,496).
Meanwhile, an optical system adapted to correct image shake by displacing a part of lens units is discussed in Japanese Patent Application Laid-Open No. 7-128619.
Generally, to miniaturize a shooting optical system, it is advisable to reduce the number of lenses and to simultaneously increase refractive power of each lens unit of the shooting optical system. However, in the shooting optical system of such a configuration, lens thickness increases. This results in insufficient shortening of a lens system, and makes it difficult to correct various aberrations.
When lens units are accommodated in a camera by being collapsed thereinto while the camera is not used, mechanical and structural errors due to inclination of a lens or a lens unit increase. In a case where the sensitivity versus eccentricity of each lens or lens unit (ratio of an amount of displacement of an image to an amount of eccentricity (or inclination) of each lens or lens unit) is high, degradation of optical performance and image shake during zooming are caused. Therefore, it is useful to reduce the sensitivity versus eccentricity of each lens or lens unit as much as possible in the shooting optical system.
Although the zoom lens discussed in Japanese Patent Application Laid-Open No. 10-62687 is suitable for a collapsible structure, a first lens unit is stationary during zooming. Thus, it is difficult to reduce the overall length of the lens when the focal length of the lens is set at a wide-angle end, and to decrease the diameter of a front lens thereof.
In the zoom lens discussed in Japanese Patent Application Laid-Open No. 2001-194586, an amount of movement of a first lens unit from a wide-angle-end to a telephoto end during zooming is small. Thus, it is difficult to sufficiently reduce the size of an entrance pupil when the focal length of the lens is set at the wide-angle end, and to decrease the diameter of a front lens thereof.
The zoom lens discussed in Japanese Patent Application Laid-Open No. 2003-315676 obtains a zoom ratio of about 5 by appropriately determining a load of varying a focal length, which is imposed onto a third lens unit from an object side. To realize a higher zoom ratio in the zoom lens discussed in Japanese Patent Application Laid-Open No. 2003-315676, thus it can be useful to share the load between the third lens unit and a second lens unit.
The zoom lens discussed in Japanese Patent Application Laid-Open No. 2004-94233 determines a moving path (locus) of a fourth lens, which is caused by zooming, to reduce variation of an entrance pupil and to decrease the diameter of a front lens.
To realize a higher zoom ratio in the zoom lens discussed in Japanese Patent Application Laid-Open No. 2004-94233, the overall length of the lens increases.